Vitamin D deficiency in older people may double the risk of developing dementia and Alzheimer’s disease, according to the largest study of its kind.

The research, published in the online medical journal of the American Academy of Neurology, found those with low levels of vitamin D in their blood (25-50 nanomole per litre) were 53% more likely to develop dementia, while those severely deficient (less than 25 nmol/L) had a 125% increased risk, compared to those with normal levels (more than 50 nmol/L).

Meanwhile for Alzheimer’s disease, people with lower levels of the vitamin were almost 70% more likely to develop the disease while this rose to 120% for those severely deficient.

The researchers looked at blood levels of vitamin D in 1,658 people aged over 65 who did not have dementia, cardiovascular disease or a history of strokes at the beginning of the trial. After an average follow-up of 5.6 years, 171 of the participants had developed dementia and 102 Alzheimer’s disease. The samples were taken from a US population–based cardiovascular study conducted between 1992–1993 and in 1999.

The study’s author, Professor David Llewellyn, said this association was twice as strong as he and his colleagues had expected.

Adding to the debate

“Clinical trials are now needed to establish whether eating foods such as oily fish or taking vitamin D supplements can delay or even prevent the onset of Alzheimer’s disease and dementia. We need to be cautious at this early stage and our latest results do not demonstrate that low vitamin D levels cause dementia.

“That said, our findings are very encouraging, and even if a small number of people could benefit, this would have enormous public health implications given the devastating and costly nature of dementia,” he said.

The researchers, from the UK's University of Exeter Medical School, the French Angers University Hospital and Florida International University, Columbia University, University of Washington, University of Pittsburgh, Veterans Affairs Ann Arbor Center for Clinical Management Research and University of Michigan in the US said their conclusions added to an ongoing debate about the role of vitamin D in non-skeletal conditions.